An ambulatory medical device, such as an implantable medical device (IMD), can be configured for implant in a subject, such as a patient. An IMD can be configured to be coupled to a patient's heart such as via one or more implantable leads. Such an IMD can obtain diagnostic information or generate therapy to be provided to the patient, such as via the coupled implantable lead. Examples of such devices can include cardiac function management (CFM) devices including one or more of implantable pacemakers, implantable cardioverter-defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), or one or more other devices. Such devices can include one or more electrodes coupled, such as via the implantable lead, to circuitry located on or within the IMD. Such circuitry can be configured to monitor electrical activity, such as to obtain information indicative of electrical activity of the heart.
A cardiac electrotherapy device to measure cardiac contractions using an elongated lead body that forms a high frequency transmission line is mentioned in U.S. Pat. No. 5,693,074 entitled “Cardiac Electrotherapy Device for Cardiac Contraction Measurement.”
A time domain reflectometry impedance sensor for measuring body impedance along a lead or catheter implanted in a patient's cardiovascular system is mentioned in U.S. Pat. No. 5,361,776 entitled “Time Domain Reflectometer Impedance Sensor Method of Use and Implantable Cardiac Stimulator Using Same.”